The invention relates to aqueous dispersions of modified epoxy resins, which are suitable as binders for aqueous coating Compositions. They are particularly used in the preparation of primer and/or primer surfacer layers in vehicle and in industrial coating.
Aqueous dispersions of epoxy resins are known in the prior art.
DE-A-3643751 for example describes aqueous dispersions of epoxy resins, which are obtained by reaction of epoxy compounds having at least two epoxy groups per molecule with an aromatic polyol and a condensation product of an aliphatic polyol and of an epoxy compound.
In EP-A-610068 epoxy resins for aqueous epoxide/amine-systems are described, which contain 20-70 wt-% of an epoxy resin and 5-40 wt-% of a self-emulsive epoxy resin, whereby the latter is obtained by reaction of a urethane compound, prepared from a polyalkylene polyether polyol and a polyisocyanate in excess, with an epoxy compound having hydroxyl groups and an alkylphenol ethoxylate.
EP-A-346 742 describes water-dilutable epoxy resins, obtained by reaction of epoxy compounds having at least two epoxy groups per molecule with an aromatic polyol and a condensation product of an aliphatic polyol, an epoxy compound and a mono- or diisocyanate, whereby the condensation product is present in the epoxy resin at 3-15 wt-%.
Further, water-dilutable epoxy resins, which are reaction products of a diglycidyl ether of a dihydric phenol, a dihydric phenol, a diglycidyl ether of a polyoxyalkylene glycol and a diisocyanate, are disclosed in U.S. Pat. No. 4,399,242. Additionally the water-dilutable epoxy resin contains an aliphatic monoepoxide as a reaction diluent.
The mentioned aqueous dispersions of epoxy resins of the prior art can be used in coating compositions, for example in combination with polyamine curing agents in aqueous two-component coating compositions. It is, however, a disadvantage of the aforementioned dispersions of epoxy resins that as binder components in aqueous two-component coating compositions lead to coating formulations which do not have the same high level of quality as the corresponding solvent-based coating formulations. In particular the poor sandability of the obtained coatings has shown to be of considerable disadvantage in the wet as well as in the dry state. The over long drying times, in particular the insufficient through-drying and the unsatisfactory final hardness of the obtained coatings are likewise unfavourable. The adhesion to the substrate is also often unsatisfactory.
The object of the invention was therefore to provide aqueous dispersions of epoxy resins, which, in combination with cross-linking agents, in particular with amino-functional cross-linking agents, can be processed into coating compositions which give the coatings an excellent sandability in the wet as well as in particular in the dry state. The coating compositions should likewise be applicable without difficulty as thick layers and should result in coatings with a rapid through-drying and a good final hardness. A good adhesion to most varied substrates should be assured.